Permanent magnet electric motors (i.e. synchronous motors) are compact and have high efficiency, and such motors are widely used in various fields, such as industry, consumer electronics, automobiles, and so on. However, for driving a permanent magnet motor, information about the position of the rotor of the motor is necessary, and due to this a position sensor has been required.
In recent years, it has become widely practiced to eliminate this position sensor, and it has become common to utilize sensor-less control for rotational speed control or torque control of a permanent magnet motor. By implementing sensor-less control, it is possible to economize upon the costs associated with the position sensor (i.e. to eliminate the cost of the sensor itself, the cost entailed by the wiring for the sensor, and so on), and to make the entire system more compact. Moreover, there are the merits that, by making the sensor unnecessary, it becomes possible to use the system in a poor quality environment, and so on. In current practice, for sensor-less control of a permanent magnet motor, either a method is employed of performing driving of the permanent magnet motor by directly detecting an induced voltage (i.e. a voltage due to speed) generated due to rotation of the rotor of the permanent magnet motor and by taking this as positional information for the rotor, or a technique of position estimation is employed by calculating an estimate of the rotor position from a numerical model of the subject motor, or the like.
However, there are also serious problems with these methods of sensor-less control. These occur with the position detection methods during low speed operation. The majority of methods of sensor-less control that are currently implemented in practice are ones based upon induced voltage generated by the permanent magnet motor. Accordingly, when the motor is stopped or in the low speed region in which the induced voltage is small, the sensitivity decreases undesirably, and there is a possibility that the position information may become buried in noise. Various strategies for solving this problem have been proposed.
With the invention described in Patent Document #1, position information is obtained by detecting the “neutral point potential”, i.e. the potential at the connection point of the stator windings for the three phases. By detecting this neutral point potential in synchrony with the pulse voltages supplied from the inverter to the motor, it is possible to detect voltage induced due to imbalance of the inductances, and it is possible to obtain the potential change depending upon the rotor position. Due to this, the above invention is distinguished by position information being obtained during normal sine wave modulation of the voltages supplied to the motor by PWM (pulse width modulation). Here, the rotor position means the position of the permanent magnet that is installed to the rotor.
FIG. 27 is a figure showing an example of a conventional synchronous electric motor drive system, in which sensor-less motor driving is performed by detecting the neutral point potential of a permanent magnet motor. A controller 1K generates PWM signals for controlling a permanent magnet motor 4 on the basis of the value of neutral point potential that is detected. The PWM signals are inputted to an inverter 3, and the inverter 3 drives the permanent magnet motor 4 on the basis of the PWM signals.
A virtual neutral point circuit 100 is connected in parallel with the permanent magnet motor 4. In order to detect the neutral point potential of the permanent magnet motor 4, a virtual neutral point potential Vnn is extracted from the virtual neutral point circuit 100. A voltage division circuit 2 is provided in order to observe the neutral point potential Vn of the permanent magnet motor 4 while taking the virtual neutral point potential Vnn as a reference. The voltage-divided potential Vin generated by this voltage division circuit 2 is inputted to an A/D converter of the controller 1K via an insulating amplifier 101.
FIG. 28(a) is a figure showing the output waveforms for the various phases as observed from the ground line (Ni) of the inverter 3. During normal PWM operation, the output potentials for the three phases change sequentially in this manner. And, at this time, the neutral point potential Vn and the virtual neutral point potential Vnn of the permanent magnet motor 4 change as shown in FIG. 28(b). Since the impedances Z3 of the virtual neutral point circuit 100 are equal to one another, accordingly, depending upon the switched state, Vnn can assume one of the four values VDC, (⅔) VDC, (⅓) VDC, and 0. Here, VDC is the DC voltage value of the DC power supply 31 to the inverter.
On the other hand, fundamentally, Vn also changes in a similar manner to Vnn, since the impedances of the three windings for the three phases are equal. However, some influence is experienced from the magnetic flux of the magnets of the permanent magnet motor 4, and accordingly these inductance values for the three phases change slightly. As a result, the inductance values for the three phases are unbalanced and depend upon which phase (positional angle) the rotor is in, so that the value of Vn varies. The difference between this Vn and Vnn itself constitutes information about the position of the rotor, so that it is thereby possible to implement positioning without any sensor. Accordingly, it is necessary to input the difference between the signals Vn and Vnn to the controller 1K. In order to implement this, the neutral point potential Vn is observed while taking the virtual neutral point Vnn as a reference.